Known methods for accelerating projectiles generally fall into three categories: a first approach is to apply a momentum to the rear of the projectile in order to accelerate it in accordance with Newton's Second Law of Motion. Alternatively, pressure may be applied to the rear of the projectile in order to accelerate the projectile also in accordance with Newton's Second Law of Motion; and, thirdly, a projectile may be accelerated in a similar manner to a rocket in accordance with Newton's Third Law of Motion.
U.S. Pat. No. 2,783,684 (Yoler) describes a method and means for propagating a mass within a tube, by generating a shock wave which is accelerated down the length of the tube in order to impart energy to the mass. The shock wave is created by means of an electric arc generated within the tube via high voltage electrodes. Electrodes are spaced along the length of the tube, so that the electric arcs will continuously be generated as the shock wave travels down the tube, thereby maintaining the pressure behind the solid mass. It is thus clear that Yoler's method is based on applying sufficient pressure to the rear of the mass in order to apply a constant thrusting force in accordance with the second of the three principles recited above.
There is likewise described in U.S. Pat. No. 2,790,354 (Yoler et al.) a mass accelerator employing electrical energy in order to propagate a projectile at high speed within a tube. The principle employed is identical to that of the first Yoler patent cited above, in that the electrical energy is used to create vast quantities of gas which create a shock wave towards the rear of the projectile.
U.S. Pat. No. 4,590,842 (Goldstein et al.) describes a method and apparatus for accelerating a projectile within a tube by generating a high velocity, high pressure plasma jet behind the projectile. Plasma jet streams are continuously generated along the length of the tube in synchronism with the motion of the projectile, by applying a high voltage across a suitable dielectric wall. The resulting plasma jets are directed through nozzles so as to apply momentum and pressure at the rear of the projectile, in accordance with the first two phenomena described above.
Electrical means for accelerating projectiles by utilizing plasma jets are also disclosed in U.S. Pat. No. 4,715,261 (Goldstein et al.), wherein a cartridge containing a plasma source for accelerating a projectile through a gun barrel bore described. The principle is identical to that employed in the first Goldstein patent cited above, in that the plasma jet imparts energy to the cartridge by means of the transfer of pressure.
Instead of using electrical means for accelerating projectiles, it is, of course, well known that chemical propellants can be used effectively to drive projectiles in a conventional gun barrel to speeds not in excess of 2 km s.sup.-1. This upper limit on the projectile velocity which can be achieved efficiently, results from the inability of the chemcial reaction to continuously supply the necessary increasing gas flow rate which is required for a constant thrust force at the base of the projectile.
This limitation of chemical propellants in conventional guns may be overcome at least to some extent in the travelling charge gun. In such a gun, as well as the conventional initial charge, an additional propellant charge is attached to the rear of the projectile, and is ignited during the acceleration process. Thus, the additional propellant charge constitutes a travelling charge which travels with the projectile until it is completely consumed, the projectile being forwardly propelled by means of the backward thrust of the burning propellant charge, relative to the projectile, which creates a corresponding forward reactive thrust on the projectile. Normally, the projectile is accelerated from rest using conventional initiating means, ignition of the travelling charge only commencing after the projectile has travelled a predetermined distance, and has therefore acquired a minimum initial velocity.
By using this technique, it is possible to obtain higher velocities due to the combined action of both the thermal pressure produced by the hot gaseous products of combustion, and a rocket mechanism which contributes additional thrust to the projectile in accordance with the third of the phenomena described above. It has been shown theoretically that in a travelling charge gun a ballistic situation can be established in which the propellant burning rate constantly increases proportionally to the projectile velocity, so as to maintain a constant pressure in the barrel behind the projectile.
The thermal pressure towards the rear of the projectile decreases significantly only when the velocity of the projectile exceeds approximately two and a half times the speed of sound of the propellant gases. This speed is the relative difference in the velocities of the gaseous products of combustion which accelerate the travelling charge, and the gases which expand from the breech of the gun. Thus, whilst the contribution of thermal pressure to the acceleration process is limited, higher velocities may nevertheless be achieved even when this limitation is reached, by employing a rocket mechanism which can be sustained in the barrel. In principle, therefore, the travelling charge gun provides an efficient method and apparatus for accelerating a projectile in order to achieve high velocities of several kilometers per second, i.e. beyond the limits of conventional guns.
Nevertheless, travelling charge guns have not enjoyed widespread use, mainly owing to the difficulty of obtaining the required burning rates of the propellants, which rates have to be controlled continuously throughout the acceleration of the projectile.